Distortion reducing circuit



April 3, 1940. F. H. SHEPARD. JR

DISTORTION REDUCING CIRCUIT Filed March-31, 1936 5 Sheets-Sheet 2 osc/L LAroR z ofz MODULATOR ATTORNEY Patented Apr. 23, 1940 UNITED STATES PATENT OFFICE DISTORTION REDUCING CIRCUIT poration of Delaware Application March 31, 1936, Serial No. 71,941

6 Claims.

The present invention relates broadly to relay circuits employing thermionic tubes and to such circuits as are incorporated in radio signalling apparatus and the like.

In the usual radio receiver, the detector output is fed to an amplifier and/or a power output circuit from which it is fed into a reproducer which may be a loudspeaker. Certain distortions are introduced due to the characteristics of the amplifying and/or power output tubes of these circuits as well as due to irregularities in the output loading device such as electrical and mechanical resonances of the output devices for instance the loudspeaker. The resonances mentioned above appear to be accentuated in practice when the output load is fed by a tube having normally high internal impedance. The present invention contemplates reducing the apparent internal impedance of the tube by feeding back to the tube input either all or a part of the A. C. output voltage in proper phase relationship. In this way the internal resistance of the tube becomes more effective to damp out the resonant peaks of the load, that is to say,

0 the resonant peaks of the load impedance are damped out by feeding back to the relay input a predetermined percentage of the A. C. output voltage.

It is an object of the present invention to de- 30 vise various circuit arrangements for overcoming the above mentioned distortions either in whole or in part.

Broadly speaking, the invention contemplates methods of comparing a part or the whole of the amplifier output voltage with the input volt-- age and applying any departure from a definite relationship between these, as signal, preferably to an intermediate amplifier in such a manner that the discrepancy will tend to be corrected.

The invention is particularly useful in connection with circuits employing a tube or tubes having a non-linear relationship between grid potential and plate current or in circuits in which the tube output load impedance is not linear and/or not uniform at all frequencies. Therefore instead of attempting to correct the load or tube characteristic it is contemplated in accordance with the invention to feed back voltage into the grid circuit of the tube or tubes in such a manner that discrepancies between the input and output voltages create a signal which is applied to the amplifier in a sense to correct the discrepancy.

According to the invention, the amplifier acts as a governor or regulator to control or keep the output voltage equal to a definite function of the input voltage. Any departure from this definite ratio creates a signal which when fed into the amplifier creates currents in the output of the amplifier which are of such magnitude 5 and direction as to tend to correct the discrepancy.

The invention will be more readily understood by reference to the following detailed specification when read in conjunction with the drawings, the various figures of which illustrate various modifications of the invention.

In the drawings:

Figure 1 illustrates in diagrammatic form a cathode drive circuit (a circuit in which signal 15 potential developed between cathode and ground is applied as signal to the following stage) in which the output voltage is subtracted from the input voltage and the difierence applied to the grid as signal; 20

Figure 2 is a diagrammatic representation of a circuit arrangement similar to that shown in Fig. 1 except that a pentode tube is used;

Figure 3 is a diagrammatic illustration of a circuit arrangement which is essentially the same 25 as that shown in Figure 1 except that the B supply and the load have been interchanged and the circuit has been grounded at a different point;

Figure 4 illustrates in diagrammatic form the an arrangement shown in Fig. 3 adapted for use with a pentode;

Figure 5 is a diagrammatic representation of a circuit arrangement illustrating a variation of the arrangement shown in Figure 3; 35

Figure 6 illustrates the circuit arrangement shown in Fig. 5 applied to a pentode;

Figure 7 represents diagrammatically a pentode driver;

Figure 8 is a circuit representation of an arrangement which combines the advantages of the circuit arrangements shown in Figures 3 and 7;

Figure 9 is a diagrammatic illustration of an arrangement which combines the advantages of the arrangements shown in Figs. 5 and 7;

Figure 10 is a diagrammatic illustration of a modification of the arrangement shown in Figure 1 wherein the difference between the input and output voltages is amplified before being applied to the grid of the output tube;

Figure 11 illustrates in diagrammatic form a variation of the arrangement shown in Figure 10 in which the load can be placed in the plate circuit of the output tube;

Figure 12 shows in diagrammatic form a pushpull arrangement of the circuit shown in Figure 2;

Figure 13 is a diagrammatic representation of a push-pull circuit arrangement of the system shown in Fig. 4; and

Figure 14 illustrates in diagrammatic form a phase inverter stage driving a push-pull output stage consisting of a pair of output pentodes.

Referring now to Figure 1 of the drawings, tube 4 may be the last tube of an amplifier, it being understood that the output of the tube maybe fed through transform-er 9 to a loud speaker for instance. The loud speaker could be connected to terminals Hi and I I. The input to tube 4 is through transformer 3, the primary of which is provided with terminals I and 2 for connection to a source of signal energy. The grid of tube 4 is grounded through the secondary of the input transformer 3 while the cathode of the tube is grounded through a self-bias resistor 6 in combination with by-pass condenser 5 and the primary of output transformer 9. To maintain the anode of the tube positive with respect to the cathode thereof there is provided a source of direct current 8 which is interposed between the anode and ground. The anode is at ground potential insofar as A. C. is concerned since the source 8 has low impedance for A. C. In case the impedance is too high, the source may be bypassed by a suitable capacitive device such as condenser I. As connected, the arrangement shown in Fig. l is a cathode drive circuit in which the output voltage is subtracted from the input voltage and the difference is applied to the grid as signal. Thus the cathode potential tends to follow the input potential, any departure of the output voltage from the desired value creates a voltage which tends to correct the discrepancy.

Referring to Fig. 2, a common type of superheterodyne receiver is shown wherein tube I2 may be an output pentode tube which includes a cathode, a grid, a screen grid, a suppressor grid and an anode. This type of tube is advantageously used as a power output tube since the suppressor grid makes possible a large power output with high gain. In applying the invention to this type of output tube, the input is between the grid and ground while the output is between the cathode and ground. The suppressor grid is tied to the cathode preferably within the tube envelope while the screen grid is connected to the anode through an audio frequency choke I3 and to the cathode by means of an audio frequency by-pass condenser I4. The anode is connected to ground through a source of space current 8 and, as in Fig. 1, should the impedance of the source for A. C. be too high it is feasible to by-pass the source 8 by means of a condenser I. In this way the anode is substantially at ground potential insofar as A. C. is concerned. The grid is biased by means of resistor 6 which is shunted by a bypass condenser 5. It is obvious from a consideration of Fig. 2 that the invention is applied in the same manner as in the circuit arrangement shown in Fig. 1 except that in Fig. 2 advantage is taken of the increased power capabilities of an output pentode;

' Reference will now be had to Fig. 3 of the drawings wherein a tube 4 provided with anode, cathode and grid electrodes is interposed between a source of electrical energy which is adapted to be connected across input terminals I and 2 and a utilizing device which may be connected across output terminals l9 and 20. The utilizing device may be represented generally as an output load coil 2!. The input is through a transformer 3 across the secondary of which there is connected a resistor I6. The resistor I6 acts to flatten the frequency characteristic of the transformer. A source of space current is provided for maintaining the anode of tube 4 at a positive potential with respect to the cathode thereof. Tracing the input in Figure 3 it will be seen that one end of the secondary of transformer 3 is connected to the grid of tube 4. The other end of the secondary transformer 3 is connected to the cathode of the tube 4 through the following circuit: From the upper end of the secondary through condenser I'I, tap I8, upper portion of output impedance 2 I, source 8, ground, condenser 5 to the cathode of tube t. The above path is, of course, the A. C. path. The D. C. path between the grid of tube 4 and the cathode thereof is, of course, through the resistors 22 and 6. From the above, it will be noted that a portion of the output impedance 2I is in the input circuit and that, therefore, feedback is provided in proper phase between the output and input of the tube. The source has been shown generally as a battery 8 throughout the drawings, however, it is to be understood that any well known type of source may be used as for instance the output of a rectifier and filter power supply unit. The anode of tube 4 is connected to the positive terminal of source 8 through the output load coil 2!. The negative v side of the source 8 is preferably grounded and the return to the cathode of the tube is obtained by grounding the cathode through the bias resistor 6 as shown. Condenser 5 is in shunt with bias resistor 6 functions as an audio frequency bypass condenser. In order that the grid may be biased with respect to the cathode there is provided a bias return resistor 22 which connects the grid to ground or to the grounded end of bias resistor ii. In accordance with the invention the feed back is obtained by connecting the grid of tube 4 to a point of the output load coil 2| through the secondary of the input transformer 3, a condenser I7 and variable tap IS. The purpose of the variable tap I8 is to provide a means for determining what proportion of the output voltage it is desired to feed-back to compare with the input. If the tap I8 is moved so that a large proportion of the voltage is fed back, then to obtain a substantial output across terminals I9 and 2:! it would necessitate a very large input across terminals I and 2. Of course, such an arrangement would reduce distortion by a correspondingly large amount due to the subtracting action of the circuit. found to be very inconvenient to supply an input voltage sufficiently large to utilize all the voltage across the load to obtain all possible distortion reducing action, accordingly, it has been found to be better to compromise and take only part of the voltage across the load for distortion reducing action. Actually for good results it is not necessary to take advantage of all the possible distortion reducing action of the circuit and in fact in an actual set-up, extremely good results were obtained by taking only 10% of the average voltage and feeding it back to the input.

Reference should now be had to Fig. 4 of the drawings wherein the invention has been shown in connection with a superheterodyne receiver of the type wherein there is provided a combined oscillator-modulator circuit. In the arrangement shown, a portion of the output of the audio frequency amplifier is fed to the input of the output pentode tube l2 and the output of the 75:

In actual practice it has been i pentode is available across the terminals 3 and 4, the utilizing device being represented generally by the output coil 2|. The usual source of space current is provided and in Fig. 4 this is indicated generally by the battery 8. The positive terminal of the source is connected to the screen grid and through the output load 2| it is tied to the anode of the tube. The negative terminal is grounded at G. The suppressor grid is connected externally of the tube to the cathode thereof thus differing from the arrangement shown in Fig. 2 wherein,

the suppressor grid is connected to the cathode within the tube envelope. The cathode return is provided through the bias resistor B and ground G. As is usual, the bias resistor 6 is shunted by the audio frequency by-pass condenser 5. In order that the grid of the tube may be properly biased a bias return resistor 22 is provided, the latter being connected between the grid and ground or between the grid and the grounded terminal of the bias resistor as shown. Energy from the audio frequency amplifier is fed through the transformer 3 the secondary of which is provided with a shunted resistor element l6 which functions to flatten the frequency characteristics of the transformer. As in the case of Fig. 3 the feed back is obtained by connecting the grid of the tube |2 to a point of the output load through the secondary of the transformer 3 and condenser H in series. In this case, however, a potentiometer arrangement is provided for tapping the desired amount of feed back. This arrangement is preferable to tapping directly on the output coil. Thus, as shown, a resistor 23 is connected across the load coil 2| and the variable tap |8 may be varied along the resistor 23 as desired. It will be noted that the circuit arrangement shown in Fig. 4 eliminates the necessity of a choke having a D. C. resistance in the screen circuit thus permitting a greater part of the supply voltage to be utilized between the screen and cathode.

In the arrangement shown in Fig. 5 the output tube 4 has applied to it between the plate and grid thereof a voltage which is in eifect the drop across the load impedance of a driver pentode tube 25. The input to the tube 25 is across the terminals and 2 as indicated in the. drawing. The terminal I connects to the signal grid of tube 25 while the terminal 2 connects to the cathode of the tube. The cathode is grounded through a bias resistor 3| shunted by a by-pass condenser 30. A source of space current 8 shown generally as a battery is provided for both tube 25 and tube 4. The positive side of the source 8 is connected to the anode of tube 25 through the load impedance 2| and the combination choke and resistor arrangement 29 and 28 respectively. The choke and resistor combination 29 and 28 forms the load impedance for the tube 25 while the choke 2| is the load impedance for the tube 4. It should be noted that the choke 2| represents in a general manner the utilizing device such as a loud speaker or the like. The signal applied between the plate and the grid of the output tube 4 is obtained by means of the drop across the load impedance 28 and 29 of the driver pentode tube 25 and is applied through the condenser 21 which connects the anode of 25 to the grid of tube 4. In the arrangement shown the drop across the load impedance 28 and 29 is a function of the driver current which is practically independent of the driver plate voltage. The cathode of tube 4 is grounded through the bias resistor 6 and the generated bias voltage is applied to the grid of tube 4 through the grid return 26. The grid bias resistor 6 is shunted by an audio frequency by-pass condenser 5. In operation of the arrangement shown in Figure 5 part or all of the output voltage is fed back in degenerative phase to the grid of tube 4. By means of a characteristic of a well screened tube this result can be obtained in a very simple fashion. The characteristic of the screen grid tube which is made use of in this connection is that its plate current is independent of plate voltage over a wide range so that the alternating current voltage developed across the output impedance 28, 29 is determined solely by the input voltage and is not affected by the inclusion of the output voltage across impedance 2| in the plate circuit of tube 25. Thus, as is evident from a consideration of Figure 5 the grid voltage of tube 4 is simply the sum of the voltage developed at the plate of tube 25 due to input voltage and the voltage developed in 2| by the plate current of tube 4. Since the potential impressed on the grid from the output is of the same phase as the potential on the plate of tube 4 the feedback is degenerative in phase.

The circuit arrangement shown in Fig. 6 is substantially the same as the circuit shown in Fig. 5 except that in Fig. 6 the output tube comprises a pentode. The screen grid of the pentode is connected to the positive terminal of the source while the suppressor grid is connected to the cathode of the tube within the tube envelope. In this circuit as in the arrangement shown in Fig. 5 the signal applied between the plate and the grid of the output pentode tube 32 is obtained by means of the drop across the load impedance combination 28, 29 of the driver pentode tube. Here also the drop across the load impedance 28, 29 is a function of the driver current which is practically independent of the driver plate voltage. The operation of the system shown in Figure 6 so far as it relates to the feedback arrangement is the same as pointed out above in connection with the description of Figure 5.

Figure '7 illustrates in diagrammatic form a driver circuit for supplying power to drive class B output tubes. In the arrangement shown in Fig. '7, tubes 43 and 44 are pentode tubes both of which have connections between the suppressor grid and the cathode and connections between the screen grids and the source of current. In the case of tube 43 the connection from the screen grid to the source 8 is through a coil 45 Whereas in tube 44 the connection is through the coil 48. The screen electrode is also connected to the cathode through the condenser 45' in the case of tube 43 and through condenser 41 in the case of tube 44.

Referring to Figure 8, tube is an output tube having a self-bias resistor 49 b-y-passed for audio frequencies by the condenser 4|. The A. C. output voltage of this tube is fed to the A. C. load through terminals 3 and 4 The choke 5| has a high A. C. impedance and is used to supply D. C. plate current to the tube 50. The grid of tube 50 is connected to the cathode of the tube 43. The plate of tube 43 is supplied with a positive B voltage as shown and is effectively grounded for A. C. The cathode of this tube which is connected to the grid of tube 50 is returned to ground through the high impedance inductance 31 and the self-bias resistor 38 which is by-passed by condenser 39. The screen of tube 43 is returned for D. C. to the positive B voltage through inductance or impedance 46, and it is grounded for A. C. to the cathode of tube 43 by means of condenser 45. The control grid of this tube is returned to ground for D. C. through grid leak resistor 52. A. C. signal input to the grid of tube 43 is applied between the plate of the output tube 50 and the grid of the tube 43 by means of the input transformer 3. The secondary of this transformer is connected for A. C. between the grid of tube 43 and the plate of tube 55!. Condenser 49 serves as a means of isolating the D. C. on the plate of tube 53 from the grid of tube 43. The input signal voltage is applied to the primary of transformer 3 through the input terminals l and 2.

It will be seen on examining the sketch that the input voltage applied between the plate of tube 50 and the grid 33 must be of sufficient value to overcome the A. C. signal on the plate of tube 59 before any actual A. C. signal is applied between the grid and cathode of tube 43. It can be seen that any discrepancy between the input voltage and output voltage results in a signal between the grid and cathode of tube 33. This grid to cathode signal tends to correct the difference between input and output voltage. The arrangement shown in Figure 8 tends to maintain an output voltage across a load substantially equal to the input voltage. If the output voltage is across a low impedance load, however, it represents large power output compared to the power input produced by the same voltage across the high impedance in- 1 put circuit. Hence. although there may not be any amplification of voltage in the system there is a large and distortionless power amplification.

Referring to Fig. 9, tube Sill is an output tube and tube 43 is a driver tube which operates in A exactly the same manner as tubes 43 and 59 described in Fig. 8. The difference of this circuit being that the signal fed between the plate. of tube 50 and the grid of tube 43 is generated in the plate circuit or across the plate circuit load 56 and 5'! of tube 53. Tube 53 is a high impedance pentode type of tube in which the plate current is only slightly effected by the plate voltage. The cathode of this tube 53 is returned to ground through self-bias resistor 65 which is by-passed by the condenser 6 The screen of this tube 53 is returned directly to the pcsitive B voltage which is at an A. C. ground potential. Signal input to this tube 53 is applied between the control grid and ground through terminals l and 2. This A. C. signal results in a variation of plate current which creates a varying voltage across the plate resistor or plate load 56 and 51. This voltage corresponds to the voltage across the transformer 3 as described in Fig. 8. It will be noted that the arrangement shown in Figure 9 replaces the input transformer 3 of Figure 8 by an auto-transformer 511 (see Fig. 9) connected in the plate circuit of a preceding voltage amplifier tube. Insofar as the feedback features of the invention are concerned, the arrangements shown in Figures 8 and 9 are substantially the same.

Figure 10 is a multi-tube arrangement in which the A. C. input voltage across terminals I and 2 is compared to the A. C. output voltage across terminals 3 and 4. The difference between the instantaneous values of the A. C. voltages results in a signal being applied between the grid and cathode of tube 66. The voltage output of tube 66 is amplified in a conventional manner by tube 61 and is applied to the grid of tube 68 in such a manner that the output current of tube 68 is varied to buck out the difference between the input voltage across terminals l and 2! and the output voltage across terminals 3 and 4. By

introducing a high degree of amplification the discrepancy between input and output voltages can be greatly minimized.

In Figure 11 tube 9| is an output tube the A. C. plate load of which may be connected across terminals 3 and 4. The D. C. plate voltage may be supplied to the plate of the tube through the inductance 92. 9| is obtained by means of the self-bias resistor 88 which is by-passed by the condenser 85?. Tubes 80 and 90 act as amplifiers to amplify the voltage applied between the grid and cathode of tube 8d. This amplified voltage is applied to the grid of tube 9|. Potentiometer 9A is a high impedance voltage divider device placed across the output load. The signal input voltage is applied to the primary of transformer 3. The output of the secondary of transformer 3 applies signal between the slider 93 of potentiometer 9dand the grid of tube 80. It can be seen that the difference between the voltage across the secondary of transformer 3 and the voltage across the upper part of potentiometer 96 is applied as signal be tween the grid and cathode of tube 88 (the cathode of tube B ll is held at A. C. ground potential by means of by-pass condenser 83). It can be seen that it is not necessary to compare the total output voltage with the input voltage but a part of the output voltage obtained by means of an inductance capacitance or resistance divider across the load can be compared to the signal input as described above.

Figure 12 is a push-pull adaptation of Fig. 2. It can be seen from the drawing that the output of each half of the load is compared with the input voltage across each corresponding half of the resistor 222 and 223.

Figure 13 is a push-pull adaptation of the circuit shown in Fig. 4. Here again the input voltage to each tube is compared to the output voltage from that particular tube.

In Figure 14 tubes 20! and 2&2 are connected in a conventional push-pull output stage. The grids of this push-pull output stage are driven respectively through condensers 201 and 238 by the plates of the double tube 208. Input voltage fed to the grid of one triode section of tube 200 results in an increased plate current in that particular section. This increased current through the inductor 209 causes the potential of the cathodes of tube 200 to change in such a manner that signal is created between the grid and cathode of the other triode unit of tube 200 in such a manner that the current change through that triode section is almost equal and opposite to the plate current change of the first section. This effect results practically in equal and opposite voltages being applied to the grids of the tubes 20! and 252. Part of the output voltage is obtained by the slider on potentiometer 2&5. This voltage is applied through condenser 206 to the grid of the second section of the triode section of the duo tube 200. This signal is of such direction and magnitude that it opposes the change created-by the input signal to the grid of the other triode section. In conclusion it can be seen that signal to the grids of the push-pull output stage is created only by a difierence in potential between the two grids of the input of the duo purpose input tube. In this circuit the input voltage on the grid of the first triode unit of tube 200 is compared with a whole or part of the output voltage obtained from the potentiometer 2115 and applied to the grid of the second triode unit of tube 200'. This difference between the two The bias to the cathode of tube grids of tube 200 results in a signal being applied to the grids of the output tube in such a manner as to correct for the difference in potential between the two grids of tube 200. In other words, the second) grid of tube 200 should have potentials equal to but opposite those on the first grid of the tube.

In the foregoing specification there have been described what are believed to be at this time the best embodiments of the invention. It is to be distinctly understood however, that the scope of the invention is not to be confined to the embodiments shown, but what it is desired to cover by Letters Patent is set forth in the appended claims.

I claim:

1. In amplifier circuits and the like an output thermionic tube having an anode, a cathode and a grid electrode, a driver circuit for said output tube, said driver circuit having an input circuit and an output circuit, said input circuit being arranged so as to be connected to a source of energy to be amplified, said output circuit including a load impedance, means for connecting said load impedance between the grid electrode and the anode of said output tube, a source of space current for both said tubes, an output load impedance device for the output tube, a circuit connecting the anode of the output tube to the oathode thereof including in series the output impedance of the output tube, the source of space current and a grid bias resistor element, and means for applying a bias potential to the signal grid of said output tube comprising a grid return resistor element connected between the grid and the anode side of the bias resistor.

2. A circuit arrangement as described in claim 1 characterized by that the output tube comprises a pentode and by that the first named load impedance comprises a choke in parallel with a resistor element.

3. In a distortion neutralizing repeater, an output thermionic tube having an input circuit and an output circuit, said output circuit including a load impedance connected between the plate and a source of positive potential, a driver circuit for said output tube comprising a screen grid thermionic tube having an input circuit and an output circuit, said last named input circuit being adapted to be connected to a source of signal voltage to apply an input voltage to the screen grid tube, said last named output circuit including a load impedance connected between the plate of the screen grid tube and a point of the first named output circuit wherein there exists an alternating current potential in the presence of input signals, saidscreen grid tube being characterized by that its anode current is substantially independent of the anode voltage over a wide range whereby the alternating current voltage developed across said last named load impedance is determined substantially solely by the input voltage applied to the screen-grid tube and is not substantially affected by the inclusion of any part of the said first-named load impedance in its output circuit, means for connecting the input circuit of said output tube across the output circuit of the screen grid tube, whereby the voltage impressed upon the input of the output tube is determined by the voltage developed across said second named load impedance due to the input voltage and by the voltage developed across the output impedance of the output tube.

4. In a repeater circuit, an output tube having an input circuit and an output circuit, said output circuit including a load impedance, a driver circuit for said output tube comprising a screen grid tube having an input circuit and an output circuit, said last named output circuit including a load impedance device one end of which is connected to the anode of the screen grid tube, means for impressing on the other end the sum of two voltages one of them a direct current voltage and the other an alternating voltage derived from said first named load impedance, means for connecting the input circuit of the screen grid tube to a source of signal voltage and means for coupling the input circuit of the output tube to the output circuit of the screen grid tube.

5. In a distortion neutralizing repeater an output tube having an input circuit and an output circuit, a load impedance in said output circuit connected between the anode thereof and a source of positive potential, a driver circuit for said tube comprising a thermionic tube having an input circuit and an output circuit said last named. input circuit being adapted to be connected to a source of signal voltage and thereby apply an input voltage to said last named tube, said last named output circuit including an impedance connected between the anode of the last named tube and a point of said first named output circuit wherein there exists an alternating current potential in the presence of input voltage to the second named tube, means for connecting the input of the output tube across the output circuit of the second named tube whereby the voltage impressed upon the input of the output tube is determined by the voltage developed across the load resistor due to the input voltage and also by the voltage developed across the output impedance of the output tube.

6. The arrangement described in claim 5 characterized by that the impedance which is connected between the plate of the second tube and a point of the output circuit includes a resistance.

FRANCIS I-I. SHEPARD, JR. 

